concrete mixing methods and concrete mixers
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Concrete Mixing Methods and Concrete Mixers
As for all materials, the performance of concrete is determined by its
microstructure. Its microstructure is determined by its composition, its curing
conditions, and also by the mixing method and mixer conditions used to process
the concrete. The mixing procedure includes the type of mixer, the order of
introduction of the materials into the mixer, and the energy of mixing (duration and
power). To control the workability or rheology of the fresh concrete, for example,
it is important to control how the concrete is processed during manufacture. In this
overview, the different mixers commercially available will be presented together
with a review of the mixing methods.
To determine the mixing method best suited for a specific application, factors to be
considered include location of the construction site (distance from the batching
plant), the amount of concrete needed, the construction schedule (volume of
concrete needed per hour), and the cost.
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Hardware: the Mixers
There are two main categories of mixer: batch mixers and continuous mixers. The
first type of mixer produces concrete one batch at a time, while the second type
produces concrete at a constant rate. The first type needs to be emptied completelyafter each mixing cycle, cleaned (if possible), and reloaded with the materials for
the next batch of concrete. In the second type, the constituents are continuously
entered at one end as the fresh concrete exits the other end. The various designs of
each type of mixer will now be discussed.
1 Batch Mixers
Two main types of batch mixer can be distinguished by the orientation of the axis
of rotation: horizontal or inclined (drum mixers) or vertical (pan mixers). The drum
mixers have a drum, with fixed blades, rotating around its axis, while the pan
mixers may have either the blades or the pan rotating around the axis.
2 Drum Mixers
All the drum mixers have a container with a cross section similar to that shown in
Fig. 1. The blades are attached to the inside of the movable drum. Their main
purpose is to lift the materials as the drum rotates. In each rotation, the lifted
material drops back into the mixer at the bottom of the drum and the cycle starts
again. Parameters that can be controlled are the rotation speed of the drum and, in
certain mixers, the angle of inclination of the rotation axis. There are three main
types of drum mixers:
non-tilting drum;
reversing drum;
tilting drum.
The non-tilting drum mixer implies that the orientation of the drum is fixed. The
materials are added at one end and discharged at the other (Fig. 2).
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Pan Mixers
All pan mixers work on basically the same principle: a cylindrical pan (fixed or
rotating) contains the concrete to be mixed, while one or two sets of blades rotate
inside the pan to mix the materials and a blade scrapes the wall of the pan. Theshapes of the blades and the axes of rotation vary. The other element of the mixer
is the scraper. Sometimes the axis of rotation of the blades coincides with the pan
axis (single paddle mixer). Other pan mixers have the axis offset [planetary motion
mixer and counter-current motion]. In these cases, there are two rotations: the
blades rotate around their axes and around the axis of the pan. The other possibility
is to have two shafts that rotate in a synchronized manner.
Continuous Mixers
The second category of mixers is continuous mixers. As the name indicates, the
materials are continuously fed into the mixer at the same rate as the concrete is
discharged. They are usually non-tilting drums with screw-type blades rotating in
the middle of the drum. The drum is tilted downward toward the discharge
opening. The mixing time is determined by the slope of the drum (usually about
15). These mixers are used for applications that require a short working time, long
unloading time, remote sites (not suitable for ready-mix) and/or small deliveries.
Mixing Method
In describing the mixing process, the mixer hardwareis only one of several
components. The mixing processalso includes the loading method, the discharge
method,the mixing time, and the mixing energy.
Loading, Mixing, and Discharging
The loading method includes the order of loading the constituents into the mixer
and also the duration of the loading period. The duration of this period depends on
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how long the constituents are mixed dry before the addition of water and how fast
the constituents are loaded. The loading period is extended from the time when the
first constituent is introduced in the mixer to when all the constituents are in the
mixer.
Mixing Energy
The energy needed to mix a concrete batch is determined by the product of the
power consumed during a mixing cycle and the duration of the cycle. It is often
considered, inappropriately, a good indicator of the effectiveness of the mixer . The
reason that it is not a good indicator is because of the high dependence of the
power consumed on the type of mixture, the batch size and the loading method.
Mixer Efficiency
As it has been pointed out, the variables affecting the mixing method are
numerous, not always controlled, and not a reliable indicator of the quality of the
concrete produced. There is, therefore, a need for a methodology to determine the
quality of the concrete produced as an intrinsic measure of the efficiency of the
mixer. The concept of mixer efficiency is used to qualify how well a mixer canproduce a uniform concrete from its constituents.
Performance Attributes as Indicators of Efficiency
Since the macroscopic properties of concrete are affected by its composition, it is
conceivable that the homogeneity of the concrete produced could be monitored by
measuring the performance of specimens prepared with concrete taken from
different parts of the mixer or at different times during the discharge. Properties
that are often considered are
workability of the fresh concrete as defined by the slump;
density of the concrete;
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air content;
compressive strength.
A disadvantage of this method is that it is indirect. It does not directly show that
the concrete is homogeneous but only assumes that any potential inhomogeneity
affects the properties considered.
Composition as an Indicator of Efficiency
A more direct method to determine the efficiency of a mixer would be to measure
the homogeneity of the concrete. This method does not rely on an assumption
about the dependency of macroscopic properties on the concrete composition. The
measure of the concrete homogeneity can be achieved by determining the
distribution of the various solid constituents such as coarse and fine aggregates,
mineral admixtures, and cement paste throughout the mixture. However, there are
no standard tests to determine homogeneity. Nevertheless, the analysis of samples
of concrete taken in various parts of a mixer or at various times during the
discharge is usually accomplished by washing out the cement paste and then by
sieving the aggregates. By weighing the sample before and after washing out the
cement paste, the cement paste content can be estimated. The aggregates collected
after the cleaning period are then dried and sieved and their size distribution is
analyzed. Because the cement paste is washed out and determined as a whole, there
is no provision to determine the dispersion of the mineral admixtures or very fine
fillers. As demands for higher performance concretes grow, more precise methods
will be needed, such as microscopic observations by scanning electron microscope
(SEM), to measure the distribution of the mineral admixtures
Output Rate as an Indicator of Efficiency
Another indicator of the efficiency of specified mixer is the output rate. The output
rate is the amount of concrete produced per a time interval. The output rate is not a
measure of the homogeneity of the concrete produced. The output rate depends on
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the time needed to load the mixer, the mixing time, the discharge time, and the
cleaning time, if it is a batch mixer. Very often this last stage is not considered, i.e.,
cleaning is not considered part of the mixing cycle. This omission is reasonable if
the mixer is continuous or if it gets cleaned only once a day. Of course, for reasons
of economics, the output rate should be high. However, it should be understoodthat it is dangerous to base the efficiency of a mixer solely on the output rate
because there is no consideration of the quality of the concrete produced.
Mixing Energy
The mixing energy is defined as the product of the average power consumption
during the whole mixing cycle and the duration of the mixing cycle. For reasons of
economics, the mixing energy should be kept low but the quality of the concrete
should be considered first.
Wear and Tear, Cleanness
In determining mixer efficiency, the main focus has been determining the
homogeneity and the quality of the concrete produced. It was assumed that the
mixer was operating as designed by its manufacturer. But long usage of a mixerleads to wear of the blades and/or scraper, or the build-up of materials (hardened
mortar or cement paste) on the blades, the container, and/or the scraper. Wear and
build-up will change the geometry of the mixer and therefore the flow pattern of
the concrete, and may lead to changes in the concrete produced [16]. To avoid this
situation, the concrete mixer should be thoroughly cleaned at the end of each day
of operation and the blades and/or scraper changed on a regular schedule. It can be
argued that criteria for a mixer selection should include
ease of cleaning;
cost and difficulty of replacing the blades or parts;
sensitivity of the mixer to wear and tear of the blades.
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Pressure grouting
Pressure grouting involves injecting a grout material into generally isolated pore or
void space of which neither the configuration or volume are known, and is often
referred to simply as grouting. The grout may be a cementitious, resinous,or solution chemical mixture. The greatest use of pressure grouting is to improve
geomaterials (soil and rock). The purpose of grouting can be either to strengthen or
reduce water flow through a formation. It is also used to correct faults
in concrete and masonry structures. Since first usage in the 19th century, grouting
has been performed on the foundationof virtually every one of the worlds
large dams, in order to reduce the amount of leakage through the rock, and
sometimes to strengthen the foundation to support the weight of the overlying
structure, be it of concrete, earth, or rock fill. Although very specialized, pressure
grouting is an essential construction procedure that is practiced by specialist
contractors and engineers around the world.